1. Field of the Invention
The present invention relates to LED lighting devices that are driven by AC power supplies, and more particularly, to an LED lighting device that is directly driven by a commercial AC power supply.
2. Description of the Related Art
LEDs (light-emitting diodes) are known as having high light-emission efficiency. In recent years, energy savings, commercialization of high-intensity white light-emitting diodes, and price reductions have been advanced. As a result, LEDs can now be used for the purpose of lighting.
PCT Japanese Translation Patent Publication No. 2003-513453 (Patent Document 1) describes an LED used for the purpose of lighting. In Patent Document 1, a plurality of LEDs are arranged in series and in parallel to define a lattice shape, and the plurality of LEDs are driven by DC voltages. In addition, even if a failure occurs in one of the plurality of LEDs, the other LEDs are not turned off.
However, as a lighting device, it is preferable to use a commercial AC power supply. In this case, in terms of energy efficiency, it is preferable that AC voltages be directly applied to the LEDs to turn the LEDs on without converting the AC voltages into DC voltages.
Japanese Unexamined Patent Application Publication No. 2003-332625 (Patent Document 2) describes a circuit for turning on LEDs using AC voltages. Patent Document 2 discloses a technology in which an LED and a diode are connected in parallel with each other such that they have polarities opposite to each other, and in which an AC voltage is applied through a capacitor to the parallel circuit. This capacitor does not have a polarity, and a forward current is applied to the LED in only a half period of the AC voltage to turn on the LED. In this case, even if a power supply having a voltage higher than a withstand voltage of the LED, such as a commercial AC power supply, is used, a voltage drop caused by the capacitor prevents a failure in the LED.
The diode is connected in parallel with the LED such that the diode is disposed in a direction opposite to the LED. This is because a rectification of the circuit can be prevented by causing a current to flow to the diode in a half period in which the LED is not turned on. If the diode is not connected, the rectification performed by the LED causes an electric charge to be stored in the capacitor. Thus, since a forward voltage is not applied to the LED, the LED is not turned on.
Instead of the diode, an LED may be used.
For lighting LEDs using a DC power supply, a configuration in which LEDs are arranged in an array as described in Patent Document 1 is taught. With this configuration, even if one of the LEDs is disconnected and turned off, the other LEDs are not turned off. However, in the case of Patent Document 1, an AC power supply cannot be directly used. In addition, when a DC voltage obtained by simply rectifying and smoothing a commercial AC power supply is used, the voltage must be reduced to an appropriate voltage. However, for example, causing a voltage to be reduced across a resistor is not practical in terms of efficiency. In contrast, when a high voltage obtained by rectifying and smoothing a commercial AC power supply is directly applied to LEDs, a significantly large number of serially connected LEDs are required. This is also not practical. The above-mentioned problems can be solved by separately providing a high-efficiency DC power supply that emits a low voltage. However, an unwanted circuit (a DC power supply) is required, and this causes problems in terms of size and price.
When a commercial AC power supply is used as described in Patent Document 2, the above-mentioned problems do not occur. However, in Patent Document 2, a circuit for turning on an LED is merely presented. That is, in Patent Document 2, a required mechanism for turning on a plurality of LEDs for lighting or the configuration described in Patent Document 1 in which a failure occurring in one of a plurality of LEDs does not affect the other LEDs is not taught. A diode may be replaced with an LED in the configuration described in Patent Document 2. In this case, however, if one of the LEDs is disconnected and turned off, the other one of the LEDs is also turned off.
Furthermore, for white LEDs, failures due to short circuits may occur more frequently than failures due to disconnections. None of the technologies described in the patent documents discussed above addresses this problem.